At night, cartons of ready made formula. YOu could also prop feed, which is tricky to master but saved my sanity in the early days once I'd got the hang of it. I'd practise it during the day so that you don't have to blunder about at night as it takes a few goes to get the hang of it.
I would place the babies in bouncy chairs and then put a cushion/pillow to the side of them. I'd then use an assortment of towels, muslin cloths, anything else to hand to prop the bottles up. I'd hold the bottle to the side of the babies' mouth so that she'd turn her head to the side and take the teat and once she was sucking away I'd basically build a cradle of muslin cloths around it and support this with towels and cushions until it was securely rested. The muslin cloths, towels, cushions, etc. effectively acted as my 'hand', holding the bottle at the right height. I'd then do exactly the same with the other twin who was waiting, boggle-eyed with famished rage with a dummy in her mouth and the two would glug happily away. If they ever had a splutter or a cough, I was sat in front of them so I could just move the bottle away and pick them up, and when they wanted to stop they just turned their head away and the teat plopped out of their mouth. It worked really well.
It took me two or three days of fiddling about with cloths under the guidance of my maternity nurse but once I'd cracked it this actually made life SO much easier. It meant both babies could feed simultaneously and I could wind one of them without disrupting the other twin's feed (which I would have to have done had I been holding the bottles). You CAN'T leave the babies on their own, but it does mean you can sit in front of them and have your hands free to wind them as and when. During the day I'd swap about a bit and feed one in my arms whilst the other prop fed (alternating twins at feeds) but when I was really frazzled and things were tricky (as they often can be in the early days) I'd do them both at the same time. They got heaps of bonding from me at every other moment of the day.
I was taught this trick by my maternity nurse who had learnt it 20 years previously in a moment of desperation (she had twins and a husband who worked night shifts so was on her own with the babies a lot) which neatly sums up how much of twin parenting is about muddling through and doing the best you can, rather than conforming to an 'ideal' that fits to a singleton baby. Necessity is the mother of invention with twins and although it's often not particularly glamourous or orthodox, if it gets you all through the day then it has to be good. As far as I was concerned, all bets were off in terms of conventional methods and if I had to x rather than y to get through the day then so be it. The only people who were going to raise an eyebrow were people with one baby and they weren't living my life.
Prop feeding really changed things for me as it took the stress and panic out of being on my own. I still got to enjoy my babies but without the dread of the next feed. I don't think parents of singletons ever feel the awful panic just prior to a twin feed when you realise you have insufficient hands for the task ahead and have NO idea how you are going to get through the next fifty minutes.
Hope this helps. I taught it to a few other twin mothers I got to know and we all did it on the days we were on our own and it really helped.